when u see a video in you tube click on the settings on the right side of the video and change the video at the desired resolution and you are done. the setting is at auto mode so if the net speed is good the video will start at higher than 480p automatically. if the net speed is low then the video will automatically start at 480 p . u cannot change the default to one mode in you tube videos. the mode is automatically set to auto mode by you tube

well...i found a way. when i close the youtube app with 480p setting and next time open the same, it opens up at 480p. However, when i play the video at 1080p and close the app, the next time i start, it plays at 1080p by default.